ADF5610 datasheet(20/48 Pages) AD

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ADF5610

Preliminary Technical Data

Rev. PrE | Page 20 of 48

‘1’ back to ‘0’ closing the loop filter path and returning

the source of the VCO tune voltage (VT) to the charge

pump.

The new VCO frequency is now routed to the input of

the PLL, divided down and compared to the reference

frequency. The charge pump adjusts the VCO tuning

voltage as necessary to lock phase lock the new

frequency to the reference frequency.

The temperature compensated voltage that is used during

autocalibration is brought out to the VPRST pin (pin 35) of the

ADF5610. This could be useful for a real time estimate of the

junction temperature of the die however some caution should

be exercised if it is desired to continuously monitor this voltage.

This is a low noise circuit and although the internal switch has

good isolation, it could still allow external noise to be coupled

onto the precision reference circuit which would result in

unwanted spurs, sidebands and degraded phase noise.

Although the digital VCO switches are normally automatically

controlled by the ADF5610 using the autocalibration feature, the

VCO switches can also be controlled directly via Register 0x05

for testing or for special purpose operations. Other control bits

specific to the VCO are also sent via Register 0x05. Refer to the

VCO Subsystem Register Map section for further details.

As mentioned previously, during autocalibration, coarse tuning

is provided by the FSM as it selects the most appropriate band

for operation over the full operating temperature range. Fine

tuning is achieved by varactor diodes once the VCO

autocalibration bit VCO_REG 0x0[7] is reset by the FSM. This

gives control of the tuning voltage back to the phase detector

and charge pump. Since the band has been selected, a narrow

arrow voltage range on the varactor is all that is needed for

phase lock. Note that the tuning voltage stays in a narrow range

over a wide range of output frequencies.

The calibration is normally run automatically, once for every

change of frequency. This autocalibration ensures optimum

selection of VCO switch settings vs. time and temperature. The

user does not normally need to be concerned about which

switch setting is used for a given frequency because this is

handled by the autocalibration routine.

The accuracy required in the calibration affects the amount of

time required to tune the VCO. The calibration routine searches

for the best step setting that locks the VCO at the current

programmed frequency and ensures that the VCO stays locked

and performs well over its full temperature range without

additional calibration, regardless of the temperature at which

the VCO was calibrated.

Autocalibration can also be disabled, thereby allowing manual

VCO tuning. Refer to the Manual VCO Calibration for Fast

Frequency Hopping section for more information about manual

tuning.

Autocalibration Using Register 0x05

Autocalibration transfers switch control data to the VCO

subsystem via Register 0x05. The address of the VCO

subsystem in Register 0x05 is not altered by the autocalibration

routine. The address and ID of the VCO subsystem in Register

0x05 must be set to the correct value before autocalibration is

executed. For more information, see the VCO Serial Port

Interface (VSPI) section.

Automatic Relock on Lock Detect Failure

It is possible, by setting Register 0x07[13], to have the VCO

subsystem automatically rerun the calibration routine and

relock itself if the lock detect indicates an unlocked condition for

any reason. With this option, the system attempts to relock only

once.

VCO Autocalibration on Frequency Change

Assuming Register 0x0A[11] = 0, the VCO calibration starts

automatically whenever a frequency change is requested. To

rerun the autocalibration routine for any reason at the same

frequency, rewrite the frequency change with the same value,

and the autocalibration routine executes again without

changing the final frequency.

VCO Autocalibration Time and Accuracy

The VCO frequency is counted for tMMT, the period of a single

autocalibration measurement cycle.

tMMT = tXTAL × R × 2n

(1)

where:

tXTAL is the period of the external reference (crystal) oscillator.

R is the reference path division ratio currently in use, set in

n is set by Register 0x0A[2:0] and results in measurement

periods that are multiples of the PD period, tXTAL × R.

The VCO autocalibration counter, on average, expects to register

N counts, rounded down (floor) to the nearest integer, for every

PD cycle.

N is the ratio of the target VCO frequency, fVCO, to the frequency of

the PD, fPD, where N can be any rational number supported by

the N divider.

N is set by the integer and fractional register contents using

Equation 2.

N = NINT + NFRAC/224

(2)

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